I. Field of the Invention
The present invention relates to the manufacture of semiconductor devices, specifically to the deposition of dielectric thin film layers using chemical vapor deposition (CVD) or transport polymerization (TP) methods. More specifically, the invention relates to the control of deposition of by-products on the interior surfaces of semiconductor manufacturing equipment.
II. Discussion of the Related Art
Integrated circuits in general are made by the stacking and patterning of thin film layers of material in and on a substrate. Such layers can be made using CVD or TP. During CVD and TP, precursors of the thin films are processed to form reactive intermediate species that are transported into a deposition chamber. Upon making contact with a surface, these reactive intermediates adhere to the semiconductor substrate surface and polymerize with each other, forming the layer of thin film. However, as these layers are applied to the semiconductor substrate surface, they also adhere to the mechanisms used to hold the substrate and the equipment used to apply the material surrounding the substrate. This unwanted contamination of the deposition equipment must be removed periodically, and during the removal process, that the equipment be shut down. Therefore, there is loss of productive time available for deposition of thin film layers resulting in decreased throughput and increased costs of manufacturing.
Many different surfaces in the deposition chamber are affected by byproduct deposition. They include the reactants dispersion head and the interior paneling of the deposition chamber.
A. Byproduct Buildup on Surfaces of the Dispersion Head
FIG. 1 is a depiction of a prior art dispersion head device 100 with gas separator plates 104, cooling plates 108, with beveled or chamfered top sides 112, and gas injection plate 116 with a flat top surface 120. Gases containing precursors for semiconductor thin films are fed into the dispersion head through the gas injector plate 116, are distributed between gas separator plates 104 and thereafter exit through the top of the dispersion head. A chuck and semiconductor substrate (not shown) are placed over the dispersion head, where the layer of semiconductor thin film is deposited.
FIG. 2 schematically depicts elements of a prior art deposition chamber 200 of an atmospheric pressure chemical vapor deposition system with prior art dispersion head 204 and gas separation plates 208. Precursor gases are mixed in the dispersion head 204 and pass through the passageways 210 between gas separator plates 208 and thereafter flow laterally in a space 212 toward a wafer 216, which is held on a susceptor 220 maintained at elevated temperature by a heater 224. As the gases arrive at the surface of the wafer, they react with the substrate of the wafer to form a thin film bonded to the wafer surface. After passing the wafer 216, the gases are exhausted through an exhaust port 228.
One of the critical variables affecting the thickness and uniformity of this thin film is the distance between the top of the dispersion head and the wafer surface. If the dispersion head is too far away from the wafer, the rate of deposition is too low, and if the dispersion head is too close to the wafer, the resulting film has poor uniformity.
Moreover, by way of example only, when growing SiO.sub.2, some of the gases react away from the wafer surface to form a white powdery byproduct. Similarly, when other materials are deposited, byproducts can form away from wafer surfaces. The powdery byproduct can deposit on any surface exposed to the precursor gases which have low gas flow rates over their surfaces. The low gas flow rates increases the residence time of the precursors at the surfaces of the apparatus. By way of example only, one of the places this product collects is on the top horizontal surface of the gas separator plates 208. As the byproduct builds up on the top surface of these prior art plates, the distance between the effective top of the plates and the wafer decreases. This decrease in separation distance causes wafer film thickness non-uniformity. Therefore, the surfaces of the prior art gas separator plates must be cleaned often, leading to decreased service time, inefficiency of manufacture, and increased costs.
FIG. 3a shows elements of a prior art dispersion head and wafer 300. An array of gas separator plates 308 comprises a series of evenly spaced plates, defining a series of gas flow channels 310 there between. The tops of the prior art plates are flat and form 90-degree angles to the side surfaces. The wafer 316 is show above the separator plates, defining a deposition space 312. In this figure, the gas flow velocities (arrows) are plotted as a function of the distance from the centerline of the array of gas separator plates (C/L) and the distance along the tops of the plates. The flow profiles were calculated using a computational fluid dynamic model derived using Fluent/UNS 4.2.5 program (from Fluent, Inc. Centerra Resource Park, 10 Cavendish Court, Lebanon, N.H. 03766), and the directions and linear speeds of the gas are shown by the arrows. In this model, gas velocity was 3.65 cm/sec through the gas flow channels 310. The model considered 2-dimensional laminar flow for compressible flow with heat transfer, with radiative, convective and conductive heat transfer to account for gas density changes. The Reynolds number for these analyses was about 1.0.
The gas speed is indicated by the lengths of the arrows in cm/seconds, and velocities are drawn to scale, with a scaling bar representing a linear flow rate of 1.0 cm/sec. The flow profile is asymmetrical, with the highest vertical flow rates being present near the centerline (C/L). Laterally from the centerline, the gas flow turns laterally, to become more parallel with the wafer 316.
FIG. 3b shows the gas velocity profiles of an array of prior art gas separator plates 308. The gas velocities are shown by the arrows, and the length of the arrows is proportional to the speed of the gas flow. The velocity scaling bar is 3.5 cm/sec. and corresponds to the highest velocity calculated for this part of this configuration. There are three plates 308 with gas flow channels 310 between them. As the gas flows upwards between the gas separator plates and exits into the space 312 above the plates, the flow of gas is non-uniform. Near the top edges of the gas separator plates 308, gas flow is directed in a parallel fashion to the top edges, and is slow, as indicated by the short arrows in a region of low velocity 311. This region of low velocity gas flow has increased residence time for the precursor gases to make contact with the separator plates 308, and consequently, powdery byproducts can deposit in this site. Further away from the top surfaces, the vectors are directed more toward the wafer and flow is rapid, as indicated by the long arrows. The wafer is held above the gas separator plates (not shown).
FIG. 3c is another depiction of the gas velocity profile of the prior art configuration shown in FIG. 3b. The gas separator plate on the right side of the figure is closer to the centerline of the array (not shown), whereas the leftmost plate 308 is farther from the centerline, thus accounting for the unequal flow profiles above each plate. In this figure, only the gas velocities 1 cm/sec and slower are shown. This makes more apparent the gas flow in homogeneity The gas velocities at the tops of the gas separator plates 308 are low, as indicated by the short arrows immediately adjacent to the tops of the plates in the areas 311. Many of the vectors indicate gas flow speeds of less than 0.1 cm/sec. Areas 311 are the sites of powder byproduct deposition.
Because the gas flow parallel to the top surfaces 311 of the gas separator plates is slow, there is opportunity for powdery byproducts to form in the gas phase and to deposit on the top surface. As the byproducts build up on the top edges of the gas separator plates, the effective distance between the top of the dispersion head and the wafer decreases. This leads to uneven film thickness uniformity. To maintain the thin film thickness and uniformity within specified limits, this byproduct must be periodically removed. The degree of byproduct buildup and the frequency of cleaning depends on many factors. For example, for the deposition of a undoped silicon dioxide layer at a deposition rate of 1000 .ANG. per minute and a deposition temperature of 500.degree. C., the byproducts must be removed after approximately 24 microns of film has been deposited on the wafer. (see Table 1). This frequency of cleaning is required to maintain the wafer-to-wafer (WTW) non-uniformity of the deposition process within .+-.2%. Removing the byproduct buildup requires stopping wafer processing, thereby reducing wafer throughput and productivity of the equipment.
Similarly, the other surfaces of the dispersion head shown in FIG. 1 are sites for deposition of powdery byproducts. The cooling plates and gas injector plates have flat surfaces, being either constructed with right angles (FIG. 1, 120) or beveled at an angle of about 45.degree. (FIG. 1, 112). Thus, these other surfaces also must be cleaned often to prevent the undesired buildup of byproducts.
B. Regulation of Precursor Gas Flow During Deposition
Because of the uneven flow of gas across the wafer during deposition processes, there is an uneven deposition ofthin film materials. As can be seen from FIG. 3a, the flow of gas is highest in the center of the wafer 316. Therefore, this is the site of the slowest deposition of thin films. One way of increasing the rate of deposition at this site is to cool the wafer at that location. However, film properties vary with deposition temperature. Furthermore, it is very difficult to closely regulate the temperature gradient of a wafer during deposition. Therefore, the uneven deposition on semiconductor wafers has been a difficult problem to overcome.
C. Byproduct Deposition on Walls of Deposition Chamber
In addition to the dispersion head elements becoming contaminated, other surfaces in the interior of the deposition chamber also become contaminated with byproducts. These include the interior walls of the deposition chamber itself.
Prior art devices have relied upon the manufacture of equipment whose interior surfaces is chosen to avoid adherence of the precursors and/or reactive intermediates and/or polymers. This makes cleaning the surfaces relatively easy. However, it necessitates the frequent cleaning, as weakly bonded contaminants can easily migrate to the substrate surface and contaminate the thin film.
Alternatively, materials have been chosen to maximize the bonding of precursors, reactive intermediates and/or polymers. This would permit a thicker layer of contaminant to be built up without severe migration of the contaminants to the substrate surface.
These methods both suffer from the problem that different precursors, reactive intermediates and polymers have different affinities for the materials comprising the interior surfaces of the manufacturing equipment. Thus, one interior surface can inhibit bonding of one type of precursor, intermediate or polymer, whereas a different precursor or intermediate can bond more tightly. Similarly, one interior surface can promote bonding of one type of precursor, intermediate or polymer, whereas a different precursor, intermediate or polymer can bond weakly. Furthermore, it is impractical to make separate deposition equipment designed specifically for a single type of molecular species to be deposited.
Therefore, the prior art deposition equipment suffers from the deposition of byproducts on various surfaces. The buildup requires frequent cleaning of the elements of the equipment, which necessitates shutting down the equipment. This results in loss of manufacturing efficiency and increase cost of semiconductor manufacture. Therefore this invention decreases the frequency of cleaning of deposition chamber equipment and improves the quality and quantity of thin films which can be deposited.